1. Field of the Invention
This invention relates to a magnetic metal particle aggregate whose particles contain Fe and Pt as alloying elements which can be used in high-density magnetic recording media, nanoscale electronic devices, permanent magnet materials, biomolecular labeling agents, drug carriers and the like, and to a method of producing the magnetic metal particle aggregate.
2. Background Art
In order to increase the recording density of a high-density magnetic recording medium, it is necessary to reduce the bit size, which is the basic unit for recording. However, conventional media using sputter-formed films are approaching the recording density limit because of problems in areas such as thermal fluctuation, particle refinement and variance of particle size. Recently, therefore, attention has been focused on FePt-based magnetic metal nanoparticles that posses high anisotropy, exhibits strong coercivity and resists thermal fluctuation as a candidate for high-density magnetic recording medium.
Japanese Patent No. 3258295 (JPA-No. 2000-54012; herein after called Reference 1) discloses a method to synthesize such magnetic metal nanoparticles, namely a method for the synthesis of monodispersed FePt alloy particles by thermally decomposing iron pentacarbonyl and reducing platinum (II) acetylacetonate by polyol, simultaneously. On the other hand, in the Journal of Applied Physics, Vol. 87, No. 9, 1 May 2000, p.5615-5617 (Reference 2), a method of reducing metal ions using boron hydride is reported. Here, the reaction site is a water/oil type reversed micelle utilizing an octane oil phase and CTAB (cetyl trimethyl ammonium bromide) as a surfactant.
The nanometer size FePt particles obtained by these methods has a disordered fcc (face-centered cubic) crystal structure, and exhibit superparamagnetism at room temperature. These particles could be made ferromagnetic by heat-treating the same and transforming the crystal structure to an L10 ordered fct (face-centered tetragonal) type.
The heat treatment has to be conducted at or above the crystal structure transition (disordered to ordered phase) temperature (Tt), which is 500° C. or higher. During heat treatment, the particle size distribution broadens owing to particle growth caused by heat-induced coalescence among the particles. As a result, the particles consist of a mixture of single and multi-domain structures that makes them unsuitable for a high-density magnetic recording media. Therefore, in order to obtain FePt particles having ferromagnetism while maintaining their particle diameter immediately after synthesis, it is effective to coat the particles with a protective agent for preventing inter-particle coalescence or to lower Tt by some method so that the heat treatment can be conducted at lower temperature.
Denshi Zairyo (Electronic Materials) January 2002, p61-67 (Reference 3) reports that the addition of third elements such as Ag, Cu, Sb, Bi and Pb during synthesis of FePt particles by the polyol process makes it possible to reduce the crystal structure transition (from fcc to fct structure) temperature (Tt). However, the iron source used was iron (III) acetylacetonate, and not iron pentacarbonyl. Another research article appeared in Jpn. J. Appl. Phys. Vol. 42 (2003) Part 2, No. 4A, L350-L352 (Reference 4) reported on the use of polyol process for the direct synthesis of fct-FePt particles. The researchers synthesized FePt particles by heating a solution of Fe (acac)3 and Pt (acac)2 in tetraethylene glycol under stirring and refluxing at different temperatures up to the boiling point. They found that the products synthesized at reaction temperatures above 280° C. exhibited XRD peaks of 001 and 110 lattice planes corresponding to fct-FePt and concluded that the FePt particles including fct structure can be directly synthesized from liquid phase. However, the transmission electron micrographs of the FePt particles suggested that the particles were not in a dispersed state but were coalesced.